Is there a way to get back to renders that are in "Render Slots" without having to preform a render?
If you rendered to the same slot, the previous information is gone forever.

If you switched to a different slot, then the information of other slots is untouched and you can switch back and forth.

To access the rendered slots you need to be in Render Result as the source for the UV/Image window:



To save the information of any of the rendered slots, use the Save as Image option on the UV/Image editor and use OpenEXR as the format, that will keep the rendered image as scene referred data.

An alternative, is to add a File Output node in the compositor. That way every time you render you will be saving a file automatically. Note that if you render again, it will render to the same file and overwrite the previous image.